1. Field of the Invention
The present invention relates to an image heating apparatus which is applied to an image forming apparatus of a copier and a printer, etc. and in particular to an apparatus having a rotary member which generates heat due to inductive heating.
2. Related Background Art
For convenience, in an image forming apparatus such as a copier and a printer, etc., an image heating apparatus (a fixing apparatus) which implements heat-fixing a toner image onto a recording material will be described as an example.
In an image forming apparatus, an apparatus involving a thermal roller is widely used as a fixing apparatus that heat-fixes an appropriate image forming processing means such as an electrophotographic process, an electrostatic recording process, and a magnetic recording process, etc. that have caused an unfixed image (a toner image) for target image information formed and carried onto a recording material (transferring material sheet, electrofax sheet, electrostatic recording paper, OHP sheet, print paper, and a format paper, etc.) by way of a transferring method or direct method as a permanent fixed image onto a recording material.
In addition, in recent years, an apparatus of a heater contact type film heating system, that is advantageous for the purpose of pursuing energy saving or shortening of wait time, etc., has become practically available.
Moreover, an apparatus of an electromagnetic inductive heating system is likewise proposed as the one which improves thermal efficiency. Japanese Utility Model Application Laid-Open No. 51-109736 has disclosed an inductive heating fixing apparatus which causes the magnetic flux to induce current in a fixing roller and generates heat with Joule heat. This directly causes the fixing roller to generate heat in use of generation of the inductive current so as to attain a fixing process with efficiency higher than that in a fixing apparatus of a thermal roller system using a halogen lamp as a heat source.
With alternate magnetic flux by way of excitation coil as magnetic filed generating means, the cylindrical fixing film itself as an inductive heating rotary body is caused to generate heat so that a fixing process is implemented.
In FIG. 16, a horizontal sectional view of a model as an example of a fixing apparatus of an electromagnetic inductive heating system being a background art of the present invention is shown.
Reference numeral 9 denotes a cylindrical fixing film as an electromagnetic inductive heating rotary body, and has inside its thickness an electromagnetic inductive heating layer (a conductive material layer, a magnetic body layer, and a resistant body layer).
Reference numeral 11 is a film guide member having a horizontally sectional view of an approximately semicircle resembling a gutter, and the above described cylindrical fixing film 9 is fitted externally and loosely from outside this film guide member 11.
Inside this film guide member 11, magnetic field generating means comprising magnetic cores (core materials) 13a and 13b forming a T with the magnetizing coil 12 are disposed.
The reference numeral 10 denotes an elastic pressing roller, which together with the lower surface of the film guide member 11 sandwiches the fixing film 9 and forms a fixing nip section N of a predetermined width with a predetermined pressure-contact force to be in pressure-contact with each other.
Pressing roller 10 is rotary-driven counterclockwise in the direction of the arrow. Rotary operation of this pressing roller 10 gives rise to the friction force between the above described pressing roller 10 and the outer surface of the fixing film 9, which force apply the rotary force to the fixing film 9, and the above described fixing film 9 having its inner surface to slide in tight contact with the lower surface of the film guide member 11 is rotary-driven around the exterior of the film guide member 11 with a peripheral velocity approximately corresponding with the rotary peripheral velocity of the pressing roller 10 in the fixing nip section N clockwise along the arrow (the pressing roller drive system).
The film guide member 11 has its role to attain the support of the excitation coil 12 and the magnetic cores 13a and 13b as pressing means onto the fixing nip section N as well as magnetic field generating means, the support of the fixing film 9, and the stability of conveyance at the time when the above described film 9 rotates. This film guide member 11 is an insulating member that does not prevent the magnetic flux from passing, and materials enduring heavy loads are used.
The temperature of the fixing nip section N is detected by a temperature detection means 18 which are brought into contact with the fixing film 9, and based thereon, current supply toward the excitation coil 12 is controlled so that temperature control is implemented to maintain a predetermined temperature.
And in addition, under the state where the pressing roller 10 undergo rotary operation, accompanied by which the cylindrical fixing film 9 rotates around the exterior of the film guide member 11, and feeding from the excitation circuit to the excitation coil 12 gives rise to electromagnetic inducted heating on the fixing film 9 as described above and the temperature of the fixing nip section N rises up to a predetermined temperature and is subject to the temperature control, the recording material P in which an unfixed toner image t is formed in the not shown image forming means section is introduced so that the image surface is caused to face upward, that is, against the fixing film surface between the fixing film 9 of the fixing nip section N and the pressing roller 10, and is sandwiched under such a state that the above described image surface is brought into tight contact with the outer surface of the fixing film 9 so as to be conveyed inside the above described fixing nip section N together with the fixing film 9.
In this process where the fixing nip section N is being conveyed, the unfixed toner image t on the recording material P undergoes heating with the electromagnetic inducted heating of the fixing film 9 so as to under heating fixing. When the recording material P passes through the fixing nip section N, it will be separated from the outer surface of the rotary fixing film 9 and be discharge-conveyed.
In such fixing apparatus of the electromagnetic inducted system as described above in FIG. 16, in which the fixing film 9 generates heat, the heat capacity of the fixing film 9 is, in particular, small and thin in thickness and therefore the coefficient of thermal conductivity of the above described fixing film 9 in the longitudinal direction is low. Thus, in the case where a recording material with width narrower than the fixing film 9 is caused to pass through, since the heat in the portion (of the non-sheet passing portion) where the recording material does not pass through is not deprived, the temperature of the fixing film in the above described sheet passing portion will rise (thermal rise in the non-sheet passing portion).
Therefore, for example, in the case where subsequent to a small size paper such as an envelope a sheet of A-4 size undergo printing, the temperature of the non-sheet passing portion after a sheet of small size paper has past through reaches the offset temperature of the toner, and therefore there gave rise to a problem that when a sheet of paper in A4 size passes through the toner on the A4 size sheet undergoes offsetting on the fixing film and good fixing image becomes unavailable.
Thus, in order to solve this non-sheet passing portion temperature rise, there is a method in which the throughput is decreased so that the number of sheets to be fed in a minute is largely decreased, which, however, was not advisable in the present times when high-speed printing is called for.
Under the circumstances, the present applicant has proposed a theory that the rotary body is brought into contact with a thermal conductive member so that the thermal difference in the rotary body in the longitudinal direction is controlled in Japanese Patent Application Laid-Open No. 11-258939.
However, there was a case where thermal efficiency dropped with a certain length in the thermal conductive member or the temperature at an end section was caused to drop.